1. Field of the Invention
The present invention relates to a color Braun tube, and more particularly to a color cathode ray tube for decreasing the landing variation caused by the change of an external magnetic field.
2. Description of Related Art
Generally, a color cathode ray tube as shown in FIG. 1 has a panel 1 coated with R,G,B phosphors on the inner surface thereof. Panel 1 is closely mated with a funnel 2, having a neck 3 into which is sealed an electron gun 4 therein for emitting multiple electron beams 5. Electron beams 5 emitted from electron gun 4 pass through apertures of a shadow mask 7 connected to a frame 8 accurately corresponding to phosphors. Electron beams 5 land on respective phosphors 6, thereby illuminating phosphors 6. Here, the landing of electron beam is affected by external magnetic fields such as ambient terrestrial magnetic fields. This degrades color purity of a reproduced image. For this reason, a magnetic shielding plate 9 has been conventionally employed, consisting of a metal sheet having one ends connected to frame 8 and the other end extending toward the apex of funnel 2.
Japanese Patent Laid-open Publication No. sho 63-67307, as shown in FIGS. 2A-2C, discloses a magnetic shielding plate 9 that includes a plurality of openings 10 which are symmetrical with respect to the horizontal axis x or vertical axis y of the tube (see FIGS. 2B and 2C).
Conventional magnetic shielding plate 9 is formed with openings 10 which are symmetrical with respect to the horizontal axis or vertical axis of the tube for converting the vertical component magnetic field of a barrel-type magnetic field A formed by the vertical magnetic field as shown in FIG. 3 into the horizontal component magnetic field to serve as a magnetic resistance B. Therefore, the change into the vertical magnetic field is decreased by the horizontal magnetic field as shown in FIG. 4. A barrel-type magnetic field D decreased at this time reduces errant electron beam landing caused by a change in an external magnetic field or direction shift of the cathode ray tube.
However, the quantity and direction of the electron beam mislanding, occurring at panel 1 of the cathode ray tube when an external magnetic field or the direction of the cathode ray tube changes, are not symmetrical with respect to the horizontal axis or vertical axis of the tube.
In more detail, when the horizontal magnetic field is applied from left to right of the cathode ray tube (in the positive direction) as shown in FIG. 3, a barrel-type magnetic field is formed within the cathode ray tube. Also, when the horizontal magnetic field is applied from the right to left (in the negative direction), a barrel-type magnetic field is formed in an opposite direction to the barrel-type magnetic field formed under the horizontal magnetic field along the positive direction. FIG. 5 illustrates the front of the cathode ray tube, especially the corner portions thereof, by separating a difference of the magnetic field formed within the cathode ray tube into the horizontal and vertical components in accordance with the magnetic fields of the positive and negative directions. Since the vertical magnetic field component affects the mislanding in the cathode ray tube for the use in the television, the horizontal magnetic field component is not considered.
Because of problems like structural asymmetry resulting from the manufacturing error of the cathode ray tube, and asymmetry of the substance characteristic and the like in the four quadrants of the front of the cathode ray tube defined by the horizontal axis and vertical axis of the cathode ray tube, the amount of variation in the vertical magnetic field is not the same at any time where an external magnetic field changes or the direction of the cathode ray tube shifts.
Portions having greater magnetic field difference such as at f or s of FIG. 5 occur to therefore differ the quantity of the landing variation by the changed amount of the vertical magnetic field in respective quadrants. The portions having the greater amount of the changed vertical magnetic field such as at f or g have the increased quantity of the landing variation, compares to h or i of FIG. 6. In more detail, the difference of the vertical magnetic field occurring within the cathode ray tube due to the positive and negative horizontal magnetic fields is symmetrical with respect to the horizontal axis or vertical axis of the cathode ray tube. Then, the asymmetrical difference of the vertical magnetic field forces the quantity of the landing variation to be asymmetrical at respective corners of the cathode ray tube when the external magnetic field or the direction of the cathode ray tube changes.
FIG. 7 represents the number of times that a landing variation occurs in respective quadrants when the horizontal magnetic field is changed in a 24-inch cathode ray tube. Here, the number of the landing variations in of the first quadrant is the greatest. Thus, color purity of the color cathode ray tube is degraded from the portion having the greatest number of landing variations when an external magnetic field or the direction of the cathode ray tube, thereby inducing the question at issue in the quality of the cathode ray tube changes.